This simple SCR trigger was found in a 1965 General Electric SCR manual. The circuit makes use of a diac, and the transistor supports an equivalent.
This preamp version uses BC109 transistors that are older versions of the current BC549 in plastic...
This circuit was found in a 1980s documentation, using a comparator from the time. The...
I found This circuit is in a documentation of transistors from 1958. It uses germanium PNP...